Method and apparatus for implementing space frequency block coding in an orthogonal frequency division multiplexing wireless communication system

ABSTRACT

The present invention is related to a method and apparatus for implementing space frequency block coding (SFBC) in an orthogonal frequency division multiplexing (OFDM) wireless communication system. The present invention is applicable to both a closed loop mode and an open loop mode. In the closed loop mode, power loading and eigen-beamforming are performed based on channel state information (CSI). A channel coded data stream is multiplexed into two or more data streams. Power loading is performed based on the CSI on each of the multiplexed data streams. SFBC encoding is performed on the data streams for each of the paired subcarriers. Then, eigen-beamforming is performed based on the CSI to distribute eigenbeams to multiple transmit antennas. The power loading may be performed on two or more SFBC encoding blocks or on each eigenmodes. Additionally, the power loading may be performed across subcarriers or subcarrier groups for weak eigenmodes.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/601,338 filed Aug. 12, 2004, which is incorporated by reference as iffully set forth.

FIELD OF INVENTION

The present invention is related to wireless communication systems. Moreparticularly, the present invention is related to a method and apparatusfor implementing space frequency block coding (SFBC) in an orthogonalfrequency division multiplexing (OFDM) wireless communication system.

BACKGROUND

OFDM is a data transmission scheme where data is split into a pluralityof smaller streams and each stream is transmitted using a sub-carrierwith a smaller bandwidth than the total available transmissionbandwidth. The efficiency of OFDM depends on choosing these sub-carriersorthogonal to each other. The sub-carriers do not interfere with eachother while each carrying a portion of the total user data.

OFDM system has advantages over other wireless communication systems.When the user data is split into streams carried by differentsub-carriers, the effective data rate on each subcarrier is muchsmaller. Therefore, the symbol duration is much larger. A large symbolduration can tolerate larger delay spreads. In other words, it is notaffected by multipath as severely. Therefore, OFDM symbols can toleratedelay spreads without complicated receiver designs. However, typicalwireless systems need complex channel equalization schemes to combatmultipath fading.

Another advantage of OFDM is that the generation of orthogonalsub-carriers at the transmitter and receiver can be done by usinginverse fast Fourier transform (IFFT) and fast Fourier transform (FFT)engines. Since the IFFT and FFT implementations are well known, OFDM canbe implemented easily and does not require complicated receivers.

Multiple-input multiple-output (MIMO) refers to the type of wirelesstransmission and reception scheme where both a transmitter and areceiver employ more than one antenna. A MIMO system takes advantage ofthe spatial diversity or spatial multiplexing and improvessignal-to-noise ratio (SNR) and increases throughput.

SFBC is a scheme for transmitting symbols of a space diversity coding onneighboring subcarriers rather than on the same subcarier in thesuccessive time slots. The SFBC avoids the problem of fast timevariations in space time block coding. However, the channel needs to beconstant over the subcarriers that combining takes place.

SUMMARY

The present invention is related to a method and apparatus forimplementing space frequency block coding (SFBC) in an orthogonalfrequency division multiplexing (OFDM) wireless communication system.The present invention is applicable to both a closed loop mode and anopen loop mode. In the closed loop mode, power loading andeigen-beamforming are performed based on channel state information(CSI). A channel coded data stream is multiplexed into two or more datastreams. Power loading is performed based on the CSI on each of themultiplexed data streams. SFBC encoding is performed on the data streamsfor each of the paired subcarriers. Then, eigen-beamforming is performedbased on the CSI to calculate eigenbeams over multiple transmitantennas. The power loading may be performed on two or more SFBCencoding blocks or on each eigenmodes. Additionally, the power loadingmay be performed across subcarriers or subcarrier groups for weakeigenmodes.

In accordance with the present invention, a robust channel estimationcan be provided in all channel conditions, with or without channelinformation feedback, and low complexity is achieved at both transmitterand receiver. In addition, scalable solution can be used with anyantenna configuration and backward compatibility is provided withenhanced performance with 802.11a/g.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an OFDM-MIMO system implementing a closedloop mode.

FIG. 2 is a block diagram of a system implementing open loop.

FIG. 3 is a block diagram of a transmitter for depicting power loading.

FIG. 4 is a diagram of an exemplary power loading and adaptivemodulation and coding mapping between two pairs of modes.

FIG. 5 shows an example of pairing of subcarrier groups for power/bitloading.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “station” (STA) includes but is not limitedto a user equipment, a wireless transmit/receive unit, a fixed or mobilesubscriber unit, a pager, or any other type of device capable ofoperating in a wireless environment. When referred to hereafter, theterminology “access point” (AP) includes but is not limited to a Node-B,a base station, a site controller or any other type of interfacingdevice in a wireless environment.

The present invention will be described with reference to the drawingfigures wherein like numerals represent like elements throughout. Itshould be noted that figures provided in the present invention are highlevel functional block diagrams and the functions implemented by thefunctional blocks may be implemented by more or less blocks. Thefeatures of the present invention may be incorporated into an integratedcircuit (IC) or be configured in a circuit comprising a multitude ofinterconnecting components.

Embodiments of the present invention provide a transmitter implementingSFBC MIMO coding and receiver matched filter. Embodiments also providetransmitter channel precoding and receiver antenna processing as well aschannel decomposition functions.

There are two modes of operation of the system: a closed loop and anopen loop. The closed loop is used when channel state information (CSI)is available to the transmitter. The open loop is used when CSI is notavailable. A variant may be used for transmission to legacy STA where itprovides diversity benefits.

In the closed loop mode, CSI is used to create virtual independentchannels by decomposing and diagonalizing the channel matrix and byprecoding at the transmitter. Given the eigenvalue spread of TGnchannels the present invention employs a space-frequency orthogonal MIMOcoding in the transmitter at the input to the channel precoder toincrease robustness at the cost of decreasing data rate. Any codingscheme in MIMO has to deal with the diversity versus multiplexing gaintrade off. It is desirable to have a trade off scheme that is bestsuited to particular channel statistics. An SFBC is chosen due to lowmobility and the long coherence time of the channel. This scheme allowsfor receiver implementation simpler than a MMSE receiver. The combinedsolution enables higher throughput over a larger range. Embodiments ofthe present invention allow for per subcarrier power/bit loading andmaintains a sustained robust link through closed loop operation withchannel state feedback. Another potential benefit is that it is easilyscalable to any number of antennas at both transmitter and receiver.

The CSI can be obtained at the transmitter either by feedback from thereceiver or through exploiting channel reciprocity. Channel reciprocityis useful for mainly TDD based systems. In this case it is possible forthe transmitter and receiver to independently estimate and decompose thechannel. The channel update rate can be lowered when the SNR is highresulting in a reduced feedback bandwidth load. Latency requirements andfeedback data rates are typically not significant to the inherentfrequency non-selectivity of eigenvalues.

The closed loop mode requires calibrations of the transmitter tocompensate amplitude and phase difference of the estimated channels inthe uplink and downlink directions. This is done infrequently, forexample during STA association or under application control, and can usechannel reciprocity for the estimation of the channel at both ends. Inaddition, a CQI (or SNR) per eigen-beam is fed back to the transmitterto support adaptive rate control.

FIG. 1 is a block diagram of an OFDM-MIMO system 100 implementing aclosed loop mode. The system 100 comprises a transmitter 110 and areceiver 130. The transmitter 110 comprises a channel encoder 112, amultiplexer 114, a power loading unit 116, a plurality of SFBC encodingunits 118, a plurality of serial-to-parallel (S/P) converters 120, aplurality of eigen-beamformers 122, a plurality of IFFT units 124 and aplurality of transmit antennas (not shown). The channel encoder 112encodes data preferably in accordance with a channel quality indicator(CQI) which is sent from the receiver 130. The CQI is used to determinea coding rate and modulation scheme per sub-carrier or group ofsub-carriers. The coded data stream is multiplexed by the multiplexer114 into two or more data streams.

The transmit power level of each data stream is adjusted by the powerloading unit 116 based on feedback. The power loading unit 116 adjustspower levels with respect to the data rate of each eigenbeam to balancethe total transmit power over all eigenbeams (or sub-carriers), whichwill be explained in detail below.

The SFBC encoding units 118 perform SFBC encoding on the data streams.SFBC encoding is done over eigen-beams and sub-carriers for each datarate that is transmitted. Eigen-beam and sub-carrier pairs are selectedto ensure independent channels. OFDM symbols are carried on Ksub-carriers. To accommodate SFBC, the sub-carriers are divided into Lpairs of sub-carriers (or group of sub-carriers). The bandwidth of eachgroup of sub-carriers should be less than the coherence bandwidth of thechannel. However, when combined with eigen-beamforming this restrictionis relaxed due to the frequency insensitivity of the eigen-beams.

The pairs of sub-carrier groups used by the block code are consideredindependent. The following is an example of the Alamouti type SFBCapplied to an OFDM symbol: $S = {\begin{bmatrix}s_{1} & {- s_{2}^{*}} \\s_{2} & s_{1}^{*}\end{bmatrix}.}$

Once the SFBC encoding units 118 construct OFDM symbols for allsub-carriers, the coded blocks are multiplexed by the S/P converters 120and input to the eigen-beamformers 122. The eigen-beamformers 122distribute the eigenbeams to the transmit antennas. The IFFT units 124convert the data in frequency domain to the data in time domain.

The receiver 130 comprises a plurality of receive antennas (not shown),a plurality of FFT units 132, eigen-beamformers 134, SFBC decoding units136, a combiner 138, a channel decoder 144, a channel estimator 140, aCSI generator 142 and a CQI generator 146.

The FFT units 132 convert the received samples to frequency domain andthe eigen-beamformer 134, the SFBC decoding unit 136 and a channeldecoder 144 perform the opposite operation which is performed at thetransmitter 110. The combiner 138 combines the SFBC decoding resultsusing maximal ratio combining (MRC).

The channel estimator 140 generates channel matrix using a trainingsequence transmitted from the transmitter and decomposes the channelmatrix into two beam-forming unitary matrices U and V, (U for transmitand V for receive), and a diagonal matrix D per sub-carrier (or persub-carrier group) by singular value decomposition (SVD) or eigenvaluedecomposition. The CSI generator 142 generates CSI from the channelestimation results and the CQI generator generates a CQI based on thedecoding results. The CSI and the CQI are sent back to the transmitter110.

The channel matrix H between nT transmit antennas and nR receiveantennas can be written as follows: $H = \begin{bmatrix}h_{11} & h_{21} & \ldots & h_{1,{nT}} \\h_{21} & h_{22} & \ldots & h_{2,{nT}} \\\quad & \quad & ⋰ & \vdots \\h_{{nR},1} & h_{{nR},2} & \ldots & h_{{nR},{nT}}\end{bmatrix}$

The channel matrix H is decomposed by SVD as follows:H=UDV^(H),where U and V are unitary matrices and D is a diagonal matrix.UεC^(nRxnR) and Vε C^(nTxnT). Then, for transmit symbol vector s,transmit preceding is simply performed as follows:x=Vs (transmitted signal).

The received signal becomes as follows:y=HVs+n,where n is the noise introduced in the channel. The receiver completesthe decomposition by using a matched filter:V^(H) H^(H)=V^(H)VD^(H)U^(H)=D^(H)U^(H).

After normalizing channel gain for eigenbeams, the estimate of thetransmit symbols s becomes $\begin{matrix}{\hat{s} = {{\alpha\quad D^{H}U^{H}{HVs}} + \eta}} \\{= {s + \eta}}\end{matrix}.$

s is detected without having to perform successive interferencecancellation or MMSE type detector. D^(H)D is a diagonal matrix that isformed by eigenvalues of H across the diagonal. Therefore, thenormalization factor α=D⁻². U are eigenvectors of HH^(H), V areeigenvectors of H^(H)H and D is a diagonal matrix of singular values ofH (square roots of eigenvalues of HH^(H)).

FIG. 2 is a block diagram of a system 200 implementing open loop mode inaccordance with the present invention. The system 200 comprises atransmitter 210 and a receiver 230. In the open loop mode, a combinationof space-frequency coding and spatial spreading in the transmitter 210provides diversity without requiring CSI. A variant of this scheme canbe used when operating with legacy 802.11a/g STAs.

The transmitter 210 comprises a channel encoder 212, a multiplexer 214,a power loading unit 216, a plurality of SFBC encoding units 218, aplurality of serial-to-parallel (S/P) converters 220, a beamformernetwork (BFN) 222, a plurality of IFFT units 224 and a plurality oftransmit antennas 226. As in the closed loop mode, the channel encoder212 uses CQI to determine coding rate and modulation per sub-carrier orgroup of sub-carriers. The coded data stream is multiplexed by themultiplexer 214 into two or more data streams.

In the open loop, the eigen-beamformer is replaced with the Beam FormingNetwork (BFN) 222. The BFN 22 forms N beams in space, where N is thenumber of antennas 226. The beams are pseudo-randomly constructed by theBFN matrix operation. The independent sub-carrier groups used for theSFBC coding are transmitted on individual beams.

For legacy support, SFBC coding may not be performed. Instead diversitythrough beam permutation is performed which improves diversity andtherefore the performance of legacy 802.11a/g equipment.

The receiver 230 comprises receive antennas 231, FFT units 232, a BFN234, an SFBC decoding and combining unit 236 and a channel decoder 238.The FFT units 232 convert the received signal in time domain to thesignal in frequency domain. The SFBC decoding and combining unit 236decodes and combines symbols received from sub-carriergroups/eigen-beams and converts them from parallel to serial using aprior knowledge of the constellation size. Symbols are combined usingMRC. The channel decoder 238 decodes the combined symbol and generates aCQI.

A first embodiment of power loading is explained hereinafter. Thespatial processing is a combination of space-frequency coding andeigen-beamforming. This is performed to give the best compromise betweenthe redundancy gains that SFBC affords and the spatial multiplexing thatthe eigen-beamformer provides. The power loading scheme operates acrossthe eigen-modes of the channel matrix. However, SFBC also introduces theconstraint that the outputs of the coder have the same power loading nomatter what the input power loading is due to the cross-operation insidethe coder.

FIG. 3 is a block diagram of a transmitter 110 for depicting powerloading. FIG. 3 illustrates 4×4 case as an example and the firstembodiment of the power loading scheme will be explained with referenceto 4×4 case. However, it should be noted that the 4×4 case can beextended to any other cases.

For a particular subcarrier k, four streams of data are mapped to 2pairs of power loading/AMC modes. In other words the modulation order isselected the same for each pair of inputs. This is later mapped to pairsof eigenmodes. Output of the power loading unit 116 is applied to thedual 2×2 SFBC encoding units 118 and then passed on to theeigen-beamformer 122. The eigen-beamformer 122 maps the inputs to theeigen-modes of the channel through the preprocessing.

For all K subcarriers, the eigenvalues of the channel matrix are knownat the transmitter. The channel energy for each eigenmode is defined asfollows:${\alpha_{i} = {\sum\limits_{k = 1}^{K}{\lambda_{i,k}}^{2}}},$where λ_(i,k) is the i-th eigenvalue for the k-th subcarrier's channel.Two SNIRs are defined for two coupled eigenmodes as follows:$\beta_{{mod}\quad 1} = {{\sum\limits_{i = 1}^{M/2}{{\alpha_{i}}^{2}\quad{and}\quad\beta_{{mod}\quad 2}}} = {\sum\limits_{i = {{M/2} + 1}}^{M}{\alpha_{i}}^{2}}}$where M is the number of eigenmodes. In other words, the eigenmodes aregrouped such that half of the eigenmodes with the largest channel energy(or SNIR) are in one group and the other half with the weakest channelenergies are in the other. Therefore, the harmonic SNIRs represent thetotal channel energy of the stronger and weaker eigenmodes. Channelenergy is an indication of how robust the eigenmodes and hence thesignal that is carried over these eigenmodes would be. This informationis used to apply different adaptivemodulation and coding (AMC) and/ordifferent power loading for each half as is explained in more detailsubsequently. The separation of the coupled SNIRs are defined asfollows:Δ_(β)=β_(mod 1)−β_(mod 2)

During the closed loop operation the transmitter 110 has the knowledgeof current CSI from which it extracts the eigenvalues and preprocessingmatrix. The transmitter 110 also infers the data rate that can besupported in the link, Rb, from the CSI. Then, power loading for agiven, acceptable, CQI is an optimization between the number of bitsthat can be sent per OFDM symbol and the type of modulation that is tobe used for each mode.

Using the channel energy calculated for eigenmode i as explained above,the maximum bit rate that can be supported for the channel condition isdetermined. Then, using the mode separation calculation above it isdetermined how the bit rate needs to be distributed between the twopairs of modes. FIG. 4 is a diagram of an exemplary power loading andadaptive modulation and coding mapping between two pairs of modes. Inthis example, the bit rate that can be supported is 24 bits per OFDMsymbol for the particular sub-carrier. The lowest modulation ordersatisfying the bit rate is found in FIG. 4 as indicated by the dashedarrow. In this example, first and second modes (first pair of coupledmodes) will be using 16 QAM and third and fourth modes (second pair ofcoupled modes) will be using 256 QAM.

Note that this mapping is described for one CQI that is acceptable andfor one subcarrier. In the case of alternative MIMO configurations, suchas 2×4, 2×2, etc, the same power loading scheme is applicable exceptthat the total number of bits in the table entries are scaled down torepresent the transmit capability and that power loading can be done ona single pair of modes.

A power loading scheme in accordance with a second embodiment isexplained hereinafter. The eigenvalues per subcarrier (λ₁(k)>λ₂(k)> . .. >λ_(nT)(k)) are ranked and eigenbeams (E¹, E², . . . , E^(nT)) arecreated by grouping the same ranked eigenvalues for all subcarriers asfollows:E ^(i)={λ_(i)(1),λ_(i)(2), . . . , λ_(i)(K)} for i=1, 2, . . . , nT,where K is the number of subcarriers, nT is the number of transmitantennas and λ_(i)(j) is the i-th eigenvalue of the j-th subcarrier. nTis an even number.

The average of the eigenvalues per eigenbeam are computed as follows:${\lambda_{av}^{i} = {{\frac{1}{K}{\sum\limits_{j = 1}^{K}{{\lambda_{i}(j)}\quad{for}\quad i}}} = 1}},2,\ldots\quad,{{nT}.}$

The eigenbeams are paired to create Alamouti space-frequency blocks,such as {E¹, E²}₁, {E³, E⁴}₂, . . . ,{E^(2i-1), E^(2i)}_(i) . . .{E^(nT-1), E^(nT)}_(nT/2). However, if the SNR of a pair is greater thanSNR_(max), then the second eigenbeam of the pair is replaced with theeigenbeam with the next lower eigenvalue average until its SNR is lessthan or equal to SNR_(min).SNR(i)=(λ_(av) ^(i)+λ_(av) ^(i+1))/σ_(n) ²,where σ_(n) ² is the noise variance and SNR_(min) is the minimumrequired SNR for the highest data rate for a required quality ofservice. This step is repeated until all the eigenbeams are paired. FIG.5 shows an example of pairing of subcarrier groups for power/bitloading.

A data rate for each pair of eigenbeams are determined by mapping theSNR of a pair to the data rate for a given quality. The required SNRsmay be adjusted for all pairs of eigenbeams to compensate for themeasurement errors and make the total transmit power be constant.

A weight vector per pair of eigenbeams per subcarrier may be computed asfollows:${{w_{k}\left( {i,j} \right)} = \sqrt{\frac{{{SNR}(i)}\sigma_{n}^{2}}{2{\lambda_{i}(j)}}}},$where i is the i-th pair of eigenbeams, j is the j-th subcarrier.

In accordance with the third embodiment, in addition to the first orsecond embodiment, another power loading is applied across thesub-carriers or group of sub-carriers for weak eigen-modes. In otherword, instead of power loading being applied to all eigenmodes it can beapplied only to those that are weaker and hence can benefit from thepower loading the most. In such a case, those eigenmodes that are notpower loaded can still have SFBC or other coding or can have differentAMC settings individually, whereas those eigenmodes that power loadedshare the same AMC setting for instance. Also, the eigenmodes of thechannel are always ordered in power, from strongest to weakest. Bypairing eigenmodes of similar power one may improve the power loading ofthe channel.

A spatial processing scheme is configurable to any number of receive andtransmit antenna combinations. Depending on the number of antennas oneach side, a combination of SFBC and eigen-beamforming options are used.The table below summarizes the various configurations supported and thestate of the spatial processing and power loading that is applicable toeach scenario. TABLE 1 Antenna Configuration Space Frequency (Tx × Rx)Block Code Eigen-Beamforming M × N (M, N ≠ 1) M/2 block codes M beams atTx N beams at Rcv 1 × N (N ≠ 1) not used To be determined by receivervendor M × 1 (M ≠ 1) M/2 block codes M beams at Tx

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

1. A method for implementing space frequency block coding (SFBC) in anorthogonal frequency division multiplexing (OFDM) wireless communicationsystem, the method comprising: performing a channel coding on input datastream; multiplexing the coded data stream to two or more data streams;obtaining channel state information (CSI); performing power loadingbased on the CSI on each of the multiplexed data streams; pairingsubcarriers for SFBC endocing; performing SFBC encoding on the datastreams; performing eigen-beamforming based on the CSI to distributeeigenbeams to a plurality of transmit antennas; and performing inversefast Fourier transform (IFFT) for converting the data stream to data intime domain for transmission.
 2. The method of claim 1 wherein thesubcarriers are divided into a plurality of groups of subcarriers. 3.The method of claim 2 wherein bandwidth of the group of subcarriers isless than coherence bandwidth of a channel.
 4. The method of claim 1further comprising: calculating channel energy for each eigenmode forall subcarriers; calculating harmonic signal-to-noise ratios (SNRs) fora plurality of modes from the channel energy; calculating separation ofthe harmonic SNRs; determining a data rate that can be supported fromthe CSI; determining bit rates distributed between the modes.
 5. Themethod of claim 4 further comprising the step of applying poweroptimization across each subcarrier or subcarrier group for weakeigenmodes.
 6. The method of claim 1 further comprising: ranking theeigenvalues per subcarrier; generating eigenbeams by grouping the sameranked eigenvalues for all sub carriers; calculating average of theeigenvalues per eigenbeam; generating space-frequency blocks by pairingthe eigenbeams; and determining a data rate for each pair of eigenbeamsby mapping required signal-to-noise ratios (SNRs) of the pair ofeigenbeams to data rates.
 7. The method of claim 6 further comprising astep of adjusting the required SNRs for all pairs of eigenbeams tocompensate for measurement errors and make a total transmit power beconstant.
 8. The method of claim 6 further comprising a step of applyinga weight vector per pair of eigenbeams.
 9. The method of claim 1 whereinthe CSI is generated by and sent back from a receiver.
 10. The method ofclaim 1 wherein the CSI is generated by a transmitter through channelreciprocity.
 11. A method for implementing space frequency block coding(SFBC) in an orthogonal frequency division multiplexing (OFDM) wirelesscommunication system, the method comprising: performing a channel codingon input data stream; multiplexing the coded data stream to two or moredata streams; pairing subcarriers for SFBC endocing; performing SFBCencoding on the data streams; generating a plurality of beams by abeamforming network and permuting the generated beams; and performinginverse fast Fourier transform (IFFT) for converting the data stream todata in time domain for transmission.
 12. An apparatus for implementingspace frequency block coding (SFBC) in an orthogonal frequency divisionmultiplexing (OFDM) wireless communication system, the apparatuscomprising: a channel coder configured to perform a channel coding oninput data stream; a multiplexer configured to multiplex the coded datastream to two or more data streams; a power loading unit configured toperform power loading based on channel state information (CSI) on eachof the multiplexed data streams; a plurality of SFBC encoding unitsconfigured to perform SFBC encoding on the data streams for each pair ofsubcarriers; a plurality of eigen-beamformers configured to performeigen-beamforming based on the CSI to distribute eigenbeams to aplurality of transmit antennas; a plurality of inverse fast Fouriertransform (IFFT) units configured to perform IFFT for converting thedata stream to data in time domain for transmission; and a plurality ofantennas.
 13. The apparatus of claim 12 wherein the subcarriers aredivided into a plurality of groups of subcarriers.
 14. The apparatus ofclaim 13 wherein bandwidth of the group of subcarriers is less thancoherence bandwidth of a channel.
 15. The apparatus of claim 12 whereinthe power loading unit comprises: means for calculating channel energyfor each eigenmode for all subcarriers; means for calculating harmonicsignal-to-noise ratios (SNRs) for a plurality of modes from the channelenergy; means for calculating separation of the harmonic SNRs; means fordetermining a data rate that can be supported from the CSI; and meansfor determining bit rates distributed between the modes.
 16. Theapparatus of claim 15 wherein the power loading unit further comprisesmeans for applying power optimization across each subcarrier orsubcarrier group for weak eigenmodes.
 17. The apparatus of claim 12wherein the power loading unit comprises: means for ranking theeigenvalues per subcarrier; means for generating eigenbeams by groupingthe same ranked eigenvalues for all sub carriers; means for calculatingaverage of the eigenvalues per eigenbeam; means for generatingspace-frequency blocks by pairing the eigenbeams; and means fordetermining a data rate for each pair of eigenbeams by mapping requiredsignal-to-noise ratios (SNRs) of the pair of eigenbeams to data rates.18. The apparatus of claim 17 wherein the power loading unit furthercomprises a means for adjusting the required SNRs for all pairs ofeigenbeams to compensate for measurement errors and make a totaltransmit power be constant.
 19. The apparatus of claim 17 wherein thepower loading unit further comprises a means for applying a weightvector per pair of eigenbeams.
 20. The apparatus of claim 12 wherein theCSI is generated by and sent back from a receiver.
 21. The apparatus ofclaim 12 wherein the CSI is generated by a transmitter through channelreciprocity.
 22. An apparatus for implementing space frequency blockcoding (SFBC) in an orthogonal frequency division multiplexing (OFDM)wireless communication system, the apparatus comprising: a channel coderconfigured to perform a channel coding on input data stream; amultiplexer configured to multiplex the coded data stream to two or moredata streams; a power loading unit configured to perform power loadingbased on channel state information (CSI) on each of the multiplexed datastreams; a plurality of SFBC encoding units configured to perform SFBCencoding on the data streams for each pair of subcarriers; a beamformingnetwork configured to generate a plurality of beams and permuting thegenerated beams; a plurality of inverse fast Fourier transform (IFFT)units configured to perform IFFT for converting the data stream to datain time domain for transmission; and a plurality of antennas.